Welding is a manufacturing or fabrication process that bonds materials such as metals or thermoplastics by causing coalescence, a process by which two separate units grow together, fuse, or merge into a single body. The materials are joined by liquefying or plasticizing (e.g., soften without liquefying) the areas to be bonded together, generally through the application of heat and/or pressure over time, promoting coalescence of the liquefied or plasticized material, and allowing the coalesced material to cool, thereby solidifying the bond. Welding can be used, for example, to join two or more work pieces or for operating on a single work piece, for example, to repair a crack or join a member.
Spot welding using electrical resistance is generally used to join together similar metallic panels with each other. However, when applied to join dissimilar metallic panels, a brittle intermetallic compound is produced at the joint. Accordingly, the spot welding of dissimilar materials has been traditionally achieved with mechanical fasteners, such as bolts and rivets.